Owing to its outstanding physical and chemical properties, including excellent heat resistance, excellent mechanical strength and resistance to radiation, silicon carbide (SiC) has attracted attention for its potential as an environmentally rugged semiconductor material. In recent years, moreover, SiC single crystal wafers have come into increasing demand as wafers for short wavelength optical devices in the blue-to-UV spectral region, high-frequency electronic devices, high-voltage electronic devices and the like. However, no wafer production technology enabling reliable supply of large-area single crystal SiC wafers of high quality on an industrial scale has yet been established. Practical utilization of SiC has therefore been impeded notwithstanding that it is a semiconductor material with these many advantages and possibilities.
Growth of SiC single-crystal of a size suitable for fabrication of semiconductor devices is possible on a laboratory scale using, for example, the sublimation growth process (Lely process).
However, the single crystal obtained by this method is of small area and its dimensions and shape are difficult to control with high accuracy. Nor can the polytype and dopant carrier concentration of the SiC be easily controlled. On the other hand, growth of cubic single crystal SiC is being carried out by heteroepitaxial growth, i.e., growth on a wafer of a different type like silicon (Si), using chemical vapor deposition (CVD). Although large-area single crystal can be obtained using CVD, only single crystal SiC containing many stacking faults and other crystal defects can be grown because the lattice-mismatch between SiC and Si is about 20%. That is, high-quality SiC single crystal is hard to obtain.
The modified Lely process, which conducts sublimation-recrystallization using an SiC single-crystal as a seed, was developed to overcome these problems (Yu. M. Tairov and V. F. Tsvetkov, Journal of Crystal Growth, vol. 52 (1981) pp. 146-150) and is in use at many research institutions. Owing to its use of a seed crystal, the modified Lely process can control the crystal nucleation process and, by controlling the ambient inert gas pressure to around 100 Pa to 15 kPa, can control crystal growth rate with good reproducibility.
Currently, 2-inch (50.8 mm) to 3-inch (76.2 mm) SiC single crystal wafers are being cut from SiC single crystal grown by the modified Lely process and used in epitaxial growth and device fabrication. In the case of, for example, applying an SiC single crystal wafer to an electronic device such as a power device, the SiC single crystal is ordinarily doped with an n-type dopant to produce a wafer of low specific volume resistance (hereinafter sometimes called “resistivity”). The n-type dopant used is nitrogen. The nitrogen doping is performed by mixing nitrogen gas into the argon or other inert gas that is the atmosphere gas used in the modified Lely process. The nitrogen atoms act as donors (electron donors) that replace carbon atoms in the SiC single crystal.
Although use of SiC single crystal wafers to fabricate SiC power devices and the like is being vigorously pursued, the resistivity of commercially available SiC single crystal wafers is fairly high, on the order of 0.015 to 0.020 Ωcm. The size of the wafer resistance relative to the device resistance therefore cannot be ignored.